Comments and References:

From Mammal Species of the World (Smithsonian Inst. Press):

"Horses have been assigned to two different species, E. caballus (includingferus and gmelini) and E. przewalskii, but recent authors include przewalskiiin caballus; see Corbet (1978ref:194), Groves (1974 ref), and Bennett (1980ref).Groves (1971ref) and Corbet (1978ref:194) proposed that ferus replace caballus,objecting to the use of specific names based on domestic animals. Gromov andBaranova (1981ref:333-334) continued to recognizetwo species, gmelini andprzewalskii."